Patent document 1 discloses a solid fuel using porous carbon as a raw material and a process for producing the same. As shown in FIG. 3, an oil mixture containing heavy oil and solvent oil is mixed with pulverized porous carbon containing water within its pores to form a slurry, which is preheated and then fed into a tank 11. The slurry is withdrawn from the bottom of the tank 11, fed to a heat exchanger 13 by a first pump 12, and heated with a compressed vapor mixture from the compressor 14 to evaporate water within the pores of the porous carbon and a part of the oils. The heated slurry and the vapor mixture comprising water vapor and oil vapor is returned to the tank 11, wherein the slurry gathers near the bottom of the tank 11, while the vapor mixture gathers near the top. The vapor mixture is withdrawn from an upper part of the tank 11 and compressed by the compressor 14 to be increased in temperature and pressure. The vapor mixture in which temperature and pressure are increased gives heat to the slurry in the heat exchanger 13 and is condensed to water and oil. The condensate containing a mixture of water and oil is transferred to an oil/water separator 15 and separated into water and oil. The water is retained in a water storage tank 16 and discharged as wastewater from the water storage tank 16. The oil is retained in an oil storage tank 17, and is pumped out by a second pump 18 from the oil storage tank 17 to be led to external recycled oil supply equipment. In the solid fuel of patent document 1, heavy oil is deposited at places within the pores of the porous carbon where water has been, whereby prevention of spontaneous ignition of the porous carbon and increased calories as a whole are realized. In addition, the vapor mixture produced by vaporizing a part of the water and oil mixture within the pores of the porous carbon in the heating process of the slurry in the process for producing the same is heated and pressured by the compressor 14 for use as a heat source.
However, the invention disclosed in patent document 1 has the following problems: by heating a slurry comprising low-grade coal containing water and an oil mixture to or above the boiling point of water within the heat exchanger 13, the water in the low-grade coal is released as water vapor in the slurry. The water vapor generated at this time is in the form of very minute bubbles, which gives foamability to the slurry. When the slurry foams, a foam phase expands, which fills a space in an upper part of the tank 11 and may also fill the inside of the pipe leading to the compressor 14 and may be sucked by the compressor 14. Originally, the compressor 14 should be provided only with the vapor mixture, that is, a gas, but the expansion of the foam phase causes not only droplets constituting the foams but also minute coal particles to be provided together to the compressor 14. As a result, there arises the problem that not only the performance of the compressor 14 is compromised, but also failures of the compressor 14 are caused. Therefore, there has been an object of suppressing the foaming produced in the dehydration process and preventing feeding of the foam phase to compressor 14, but no specific disclosure has been made on a means for cleaning the vapor mixture.
Such bubbles have been conventionally and generally coped with by allowing them to settle naturally, but there have been the problems that a process loss is caused since the settlement of bubbles takes time and that installing a large tank for settling bubbles is costly.
Patent document 2 discloses a process for mechanically separating bubbles and liquids by using a centrifugal force as a process for preventing foaming, but it is not realistic since a huge machine is required in the case of an increased throughput.
Patent document 3 discloses a process for suppressing foaming by adding a defoamant. The process has been suggested and commercialized in various forms depending on the nature of foaming and materials, but using a large amount of expensive defoamants for fuels with low added values such as coal is economically unviable.